To a stirred solution of (R)-1-[3-(1-tert-butoxycarbonyl-4-piperidyl)propionyl]-3-piperidine carboxylic acid (1.0 g) in tetrahydrofuran (20 ml) was added dropwise isobutyl chloroformate (356 μl) and 4-methylmorpholine (300 μl) at −15° C. under a nitrogen atmosphere. To an ice cooled solution of 2(S)-benzyloxycarbonylamino-β-alanine methyl ester hydrochloride (783 mg) and N-(trimethylsilyl)acetamide (1.78 g) in tetrahydrofuran (30 ml) was added dropwise the above solution with stirring under a nitrogen atmosphere. The reaction mixture was allowed to warm to ambient temperature, and stirred for 2 hours, which was partitioned between ethyl acetate and water. The organic layer was separated, washed in turn with water, aqueous 5% KHSO4, aqueous 5% NaHCO3 and brine, and dried over MgSO4. Evaporation of the solvent gave a residue, which was purified by silica-gel column chromatography eluting with n-hexane-ethyl acetate (from 1:6 to ethyl acetate only) to give N-[(R)-1-[3-(1-tert-butoxycarbonyl-4-piperidyl)propionyl]-3-piperidylcarbonyl]-2(S)-benzyloxycarbonylamino-β-alanine methyl ester (1.36 g) as a foam, which is the same compound obtained in Preparation 7.
